The vertebral column of a human being is typically made up of thirty-three vertebrae, which provide axial support to the trunk of the body and protection for the spinal cord. The majority of the vertebrae are articulated, and are separated by intervertebral disks that act as shock-absorbers. The remainder of the vertebrae are fused together, forming the sacrum and coccyx.
FIG. 1 shows a lateral view of the human vertebral column. The vertebral column has three major sections, each of which is differently curved. In a healthy patient, the cervical spine 102 is lordotic, the thoracic spine 104 is kyphotic, and the lumbar spine 106 is lordotic.
The vertebrae have different shapes and dimensions, particularly as one proceeds down the vertebral column. However, a vertebra typically comprises a cylindrical core known as the vertebral body having a smooth rounded surface facing the anterior (front) of the person's body. Each vertebra also typically has facets or structures projecting from the posterior of the vertebra. The pedicle is an example of a stem-like structure that projects from the posterior of the cylindrical body of a vertebra. There are two pedicles per vertebra and owing to their high bone density, the pedicles are typically used as anchoring locations for surgical screws, known as pedicle screws.
There are various abnormalities relating to curvature of the vertebral column. Vertebrae can become displaced in any of three dimensions with respect to each other. Correction may be necessary when displacement occurs outside of normal bounds and/or causes problems for the individual.
For example, abnormal displacement can occur in the anteroposterior direction. That is, the curves of FIG. 1 can become over-pronounced, potentially resulting in medical problems for the patient. Kyphosis is a condition that results when there is excessive (flexion) outward curvature of the thoracic curved region (i.e. towards the posterior) giving a humped-back profile. Lordosis occurs when there is excessive (extension) inward curvature (i.e. towards the anterior) of the lumbar or cervical curves.
Curvature can also occur in the lateral (or ‘side-to-side’) direction. FIGS. 2 and 3 show anteroposterior (AP) views of different vertebral columns. Specifically, FIG. 2 shows a normal vertebral column, wherein the vertebrae are substantially aligned vertically when viewed from the anteroposterior direction (front). In contrast, FIG. 3 shows a vertebral column suffering from scoliosis (lateral curvature). It can be seen that the vertebrae in FIG. 3 are not aligned vertically, but are instead curved laterally (x-direction, as shown in the axes depicted in FIG. 3). In scoliosis, as well as lateral curvature there tends to be lordosis and rotation with the posterior elements directed towards the midline.
The most common type of scoliosis is idiopathic scoliosis, which affects many millions of people worldwide. A curvature of 10 degrees or more is present in 2 to 3% of the population. Surgery is usually recommended for curves which have a high likelihood of progression (i.e., greater than 45 to 50 degrees) and curves which are cosmetically unacceptable. Current surgical techniques most commonly use spinal implants and bone to fuse the spine in order to improve deformity and prevent curve progression. Surgery over a few hours attempts to correct a deformity which has been developing for months or years. Complete correction is rare because the bones deform in response to the abnormal forces and contractures occur because of the deformity.
The current “gold-standard” of scoliosis surgical techniques involves the implantation of hooks and screws to the spine to anchor long stabilizing rods. Problems with this approach include:                1. A large implant which runs the risk of infection.        2. Use of pedicle screws which may cause neurological damage, and may need to be removed.        3. The deformity will have been developing for many months or years. It is seldom possible to correct this deformity in the few hours of surgery.        4. Most current devices use spinal fusion. This prevents movement in the part of the spine which has been fixed. It also prevents most growth in that part of the spine.        5. Because the implants are stiff occasionally breakage of the implants can occur.        